Clean up <uchar.h> and <wctype.h> transitive includes.

Bug: http://b/30367366
Change-Id: I63212a34c54ca07223a15aa5be4da3bdc6aa38a8
diff --git a/libc/include/uchar.h b/libc/include/uchar.h
index 6b995ab..818acb1 100644
--- a/libc/include/uchar.h
+++ b/libc/include/uchar.h
@@ -29,8 +29,9 @@
 #ifndef _UCHAR_H_
 #define _UCHAR_H_
 
+#include <stddef.h>
 #include <sys/cdefs.h>
-#include <wchar.h>
+#include <bits/mbstate_t.h>
 
 __BEGIN_DECLS